Comp 430 Hardware Overview Lecture

Comp 430
Computer Graphics
Lecture 2:
Hardware Overview

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related subject.Jan 2009 - Lecture2 CSC406 Computer Graphics 2

Jan 2009 - Lecture2 CSC406 Computer Graphics 3
Lecture 2:
 Scope:
 Graphics hardware.
 Objectives:
 To determine the characteristics of graphics
display hardware.

Computer Hardware:
Output Devices:Output Devices:
 Computer Graphics can use many different
output devices:
 Video display devices
 Cathode Ray Tube (CRT)
 Liquid Crystal Display (LCD)
 Plasma panels
 Thin-film electroluminescent displays
 Light-emitting diodes (LED)

 Hard-copy devices
 Ink-jet printer
 Laser printer
 Film recorder
 Electrostatic printer
 Pen plotter
 The most common display device is the
Cathode Ray Tube (CRT) monitor.

The Cathode Ray Tubes (CRTs)
 Most common display device today
 Characterized by:
 Evacuated glass bottle
 Extremely high voltage
 Heating element (filament)
 Electrons pulled towards anode focusing cylinder
 Vertical and horizontal deflection plates
 Beam that strikes phosphor coating on front of
tube

CRT-2

How CRT works-1
 The electron gun contains a filament that,
when heated, emits a stream of electrons
 Electrons are focused with an electromagnet
into a sharp beam and directed to a specific
point of the face of the picture tube
 The front surface of the picture tube is coated
with small phospher dots
 When the beam hits a phospher dot it glows
with a brightness proportional to the strength
of the beam and how long it is hit

How CTR works - 2
 In a CRT the focusing system acts like a light lens
with a focal length such that the center of focus is
the screen.
 The horizontal and vertical deflectors allow the
electron beam to be focused on any spot on the
screen.
 The screen is coated with a special organic
compound called a phosphor.
 For color systems there are groups of three different
phosphors, one to produce red shades, one for
green shades, and one for blue shades.

How CRT works - 3
 Electrons hit the screen phosphor molecules and
cause a ground state to singlet excited state
transition.
 Most of the phosphors relax back to the ground
state by emitting a photon of light which is called
fluorescence.
 This happens very rapidly so that all of the molecules which
fluoresce do so in under a millisecond.
 These phosphors then emit light, called
phosphorescence, that decays slower but still
rapidly (in about 15-20 milliseconds)…
 so there is the need to refresh the screen by redrawing the
image.

CRT details…
 Phosphors are characterized by color (usually red,
green, or blue) and persistence, which is the time for
the emitted light to decay to 10 % of the initial
intensity.
 High persistence phosphors allow for a lower refresh
rate to avoid flicker,
 e.g., the original IBM PC monochrome monitor had a high
persistence phosphor.
 This allowed it to have good resolution for text with
inexpensive electronics.
 However, this is poor for animation since a "trail" is left with
moving objects.

CRT details…
 Low persistence phosphors are good for
animation but require a high refresh rate to
prevent flicker.
 A refresh rate of 50 - 60 Hz is usually sufficient to
prevent flicker,
 but some systems refresh at even higher rates
such as 72-76 Hz.

CRT details…
Phosphers – terms…
 Flourescence: Light emitted while the
phosphor is being struck by electrons
 Phosphorescence: Light emitted once the
electron beam is removed
 Persistence: The time from the removal of the
excitation to the moment when
phospherescence has decayed to 10% of the
initial light output

CRTs - Display Technologies…
 Two main types:
 Vector display.
 Raster display.

CRTs - Vector Displays
 Early computer displays: basically an oscilloscope
 Control X,Y with vertical/horizontal plate voltage
 Often used intensity as Z (close things were
brighter)
 Often termed random scan displays.

Vector displays:
 Here the electron gun of a CRT illuminates
points and straight lines in any order.
 The display processor repeatedly reads a
variable 'display file' defining a sequence of
X,Y coordinate pairs and brightness or color
values, and converts these to voltages
controlling the electron gun.

Vector Display…
 Advantages of random scan:
 very high resolution, limited only by monitor
 easy animation, just draw at different positions
 requires little memory (just enough to hold the display
program)
 Disadvantages:
 requires "intelligent electron beam, i.e., processor
controlled
 limited screen density before have flicker, can't draw a
complex image
 limited color capability (very expensive)

A Random Scan Display (outline)…

Display technologies: CRTs
Raster Displays
 Raster: a raster is a series of adjacent parallel 'lines'
which together form an image on a display screen.
 Pixel: One dot or picture element of the raster
 Scan line: A row of pixels

Display technologies: CRTs
Raster Displays
 Black and white television: an oscilloscope with a fixed
scan pattern: left to right, top to bottom
 As beam sweeps across entire face of CRT, beam
intensity changes to reflect brightness
 Analog signal vs. digital display

Rasters:Rasters:
 Lexically, a raster is a series of adjacent parallel
'lines' which together form an image on a display
screen.
 In early analogue television sets each such line is
scanned continuously, not broken up into distinct
units.
 In computer or digital displays these lines are
composed of independently coloured pixels (picture
elements).

A Raster

Resolution…
 The maximum number of points that can be
displayed without overlap is called the
resolution.
 Usually given as the number of horizontal
points versus the number of vertical points
 These are called pixels (picture elements),
E.g.
 a monitor might have a resolution of 1024 X 768
pixels.

 The maximum resolution may be determined
 by the characteristics of the monitor for a random
scan system or
 by a combination of the monitor and graphics card
memory for a raster scan system.
 For a random scan system the resolution can be
up to 4096 X 4096

Aspect Ratio:
 The aspect ratio equals the ratio of vertical
pixels/horizontal pixels for an equal length line.
 It sometimes refers to the ratio of the horizontal
dimension/vertical dimension.
 Examples:

 If have the above monitor and a resolution of 640 X
480 pixels
 ==> horizontal --> 640/8 = 80 pixels/inch
 ==> vertical --> 480/6 = 80 pixels/inch
 Therefore, it has "square" pixels, i.e. the same size
in the vertical and horizontal directions.
 If the resolution is 320 X 200 pixels, then:
 ==> horizontal --> 320/8 = 40 pixels/inch
 ==> vertical --> 200/6 = 33 1/3 pixels/inch
 So the size of a horizontal pixel does not equal the size of a
vertical pixel.
 We must correct for this in our image display, e.g. line or
object drawing.
 Otherwise all of the drawings will be distorted.

 Mathematically we consider a raster to be a
rectangular grid or array of pixel positions:
 Pixel positions have X,Y coordinates.
 Usually Y points down - This may reflect early use
to display text to western readers.
 When considering 3D, right-handed coordinates
imply Z - represents depth.

Pixel Values:Pixel Values:
 The colour of each pixel of a display is
controlled by a distinct digital memory
element.
 Each such element holds a pixel value
encoding a monochrome brightness or colour
to be displayed.
 With respect to color/pixel value, displays can
be classified into two broad categories:
 Monochrome.
 Non-monochrome.

Monochrome displays…
 Are of two types.
 Bi-level displays - have 1-bit pixels and have been
green or orange as well as black-and-white.
 Greyscale displays - usually have 8 to 16 bit pixel
values encoding brightness.

Non-monochrome displays…
 Also have two types.
 True-colour
 Displays have pixel values divided into three component
intensities, usually red, green and blue
 Often of 8 bits each - This used to be very costly.
 Alternatively the pixel values may index into a fixed or
variable colour map defining a limited colour palette.
 Pseudo-colour
 Displays with 8-bit pixels indexing a variable colour map
of 256 colours have been common.

Comp 430 Hardware Overview Lecture

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